Generally, image receiving devices include television, video tape recorder (to be called hereinafter "VTR") and the like, and VTR is a device which is capable of recording television image signals on a magnetic tape, and regenerating the television image signals recorded on the magnetic tape.
Among the receiving channels of television and VTR, the non-broadcasting channels which belong to the non-signal bands show ugly screens and wild noise; therefore, it is the recent trend that efforts are made to mute the sounds of the non-broadcasting channels and to furnish a blue color screen or other monochromatic screen.
Further, among the receiving channels of televisions and VTR, the non-broadcasting channels occupy a greater part of the total channels, and therefore, if there are many non-broadcasting channels between two broadcasting channels, an inconvenience is experienced in shifting from one broadcasting channel to another broadcasting channel, as well as in consuming excessive time in performing the shifting.
In order to overcome such a problem, certain kinds of televisions and VTRs are constructed to discriminate the non-broadcasting channels through utilization of the picture search function, the discriminated broadcasting channels are stored in the memory, and the non-broadcasting channels are left to be skipped, thereby saving channel shifting time.
If the non-broadcasting channels of televisions and VTRs are left free to give muted sounds and a mono-chromatic screen, and if the non-broadcasting channels are to be skipped, then the non-broadcasting channels have to be discriminated, and the discrimination is carried out in such manner that the existence of a synchronizing signal is detected by means of a synchronism detecting device, thereby recognizing the existence or absence of an image signal based on the detected result.
However, the conventional synchronism detecting circuit is liable to commit erroneous recognitions such that even the noise signals in the form of pulse arranged in a manner similar to the vertical synchronism and the horizontal synchronism are accepted as synchronizing signals, thereby judging a non-broadcasting channel as a broadcasting channel. Therefore such televisions and VTRs are imperfect in muting the noise and in furnishing a mono-chromatic screen, as well as being incapable of skipping the non-broadcasting channels during the image search.